Volumetric Lighting light pass
What you can do is to separate your scene into two layers

Volume layer: Here there will be the lights and you have to change the material of all the objects to Holdout, you have to check Transparent on the Film menu as well.

Objects layer: Here there will be all the objects with their normal materials and all the lights. Just like your normal scene, but you have to set the density of the Volume world to 0.

Then, in the compositor, you have to do the next:



You can also try the denoising feature to reduce the noise, as well as reducing the step size and Max steps for Volume Sampling in the Geometry menu.

Hope that helps